1) Field of the Invention
The present invention relates to a method for controlling driving on a hill for all-wheel drive vehicles, wherein vehicle acceleration is determined and an acceleration of the vehicle is measured.
2) Description of Related Art
Modern motor vehicles are equipped with most various electronic systems for controlling and regulating the driving behavior. Examples of these systems are brake force control systems (ABS), driving stability programs (ESP) and traction slip control systems (TCS, BTCS, or TCS). The control programs provided in the electronic unit of these systems evaluate depending on the design a large number of sensors mounted into the motor vehicle. The sensors are e.g. wheel speed sensors, longitudinal acceleration sensors, lateral acceleration sensors, and yaw rate sensors. In traction slip control, the electronic unit ensures that the wheels accelerating the vehicle will not spin at great acceleration of the vehicle. On the one hand, this can be done by slowing down the spinning wheel, and/or by throttling the driving power (engine torque reduction).
DE-A 3809101 discloses a method and a circuit arrangement for controlling a TCS system with brake and engine management. For traction slip control, the wheel brake is used and/or intervention into the control of the driving engine is made with this method. To this end, traction slip control makes use of conventional components of the already provided anti-lock system (ABS) among others. The communication with the required vehicle components (e.g. engine) can take place by way of a vehicle data bus (CAN) known in the art. To implement the method, sensors for determining the wheel rotational behavior are evaluated by circuits for signal processing purposes, and corrective signals for producing electromagnetic hydraulic valves are generated and allow controlling the brake force. The electronics of the system requires the so-called vehicle reference speed Vref to calculate the necessary correcting variable. This reference speed is supported by gradients and generally determined by the wheel speed of the slowest wheel. The gradient is determined from the current engine torque and the empty weight of the vehicle. The calculated gradient corresponds to the theoretical vehicle acceleration at a high coefficient of friction in the plane. When the all-wheel drive vehicles are equipped with an acceleration sensor (G-sensor), said sensor is additionally used for defining the gradients. The maximum of calculated and measured gradient is determined to this end. When special driving situations are encountered, it may occur that one or more of the wheels will no longer convey the current vehicle speed as one of the wheels is spinning. It is usual in this case that the spinning wheel is not taken into consideration for producing the vehicle reference speed Vref.
In addition, the problem is encountered in all-wheel drive vehicles that all wheels can exhibit traction slip so that there is no indicator of the vehicle reference speed, which is usually determined from the wheel speeds, in a case of driving, rendering it impossible to make a distinction e.g. between downhill driving and traction slip. This problem cannot occur in a case of driving in vehicles where one axle is driven because at least the non-driven axle can exhibit the absence of traction slip. Thus, the wheels of the non-driven axle can always be used as an indicator of the reference speed.
DE 197 32 554 A1 discloses a method and a device for determining the speed of an all-wheel driven vehicle. In this publication, individual wheel accelerations are related to the prevailing engine torque and compared with one another in order to be able to initially detect the condition that all wheels are spinning and then take remedies. It is disadvantageous in this method that for different reasons the detection thresholds must be chosen to be comparatively coarse to avoid errors in identification, with the result that the detection is not very precise and a road incline remains generally unnoticed.
When situations occur in which the wheel speeds alone are no longer sufficient to reliably determine the vehicle reference speed, substitute strategies are used. Substitute strategies permit solving individual problems known in the art such as when the driver rides the vehicle at a low coefficient of friction with spinning wheels or when the vehicle is accelerated under defined conditions when driving on a hill, especially when driving downhill. The vehicle reference speed determined as a substitute then becomes inaccurate at an increasing rate when conditions coincide, and there is the need to solve also problems arising when an all-wheel driven vehicle suffers from an unstable wheel run during downhill driving, for example.
The influence of driving on a hill is only inappropriately or not at all taken into consideration in the above-mentioned electronic systems for controlling the vehicle behavior. When driving down a slope, the actual vehicle acceleration is higher than the calculated vehicle acceleration [f(engine torque, vehicle mass)] and the measured vehicle acceleration (G-sensor or acceleration sensor) shows an acceleration value falsified by the gradient component. To compensate the slope, an invariable component or a fixed gradient has been added so far to the calculated gradient under defined conditions. This type of a slope correction is necessary to rule out a too low reference speed during downhill driving. Said slope correction is, however, linked to conditions that may cause shortcomings on different road surfaces.